An air conditioner for air conditioning, such as cooling and heating, as one of electric devices, generally includes a fan for blowing air and an electric motor for rotary-driving the fan. In recent years, so-called brushless motors have been used as electric motors in many cases. In such a brushless motor, an inverter of a pulse width modulation system (hereinafter, referred to as a PWM system as appropriate) is used. On the other hand, the PWM system performs switching using high-frequency pulses, and thus unnecessary high-frequency signals are likely to radiate from the brushless motor.
In order to address this problem, an air conditioner for reducing high-frequency noise radiated from such an electric motor is conventionally proposed (see Patent Literature 1, for example). The technique used in such an air conditioner is to cover the part likely to be affected by radiated noise with a shield case, and to connect the shield case with the earth cable inside the air conditioner.
When an electric motor is driven by such an inverter of the PWM system, the neutral point potential of the winding is not zero, which causes a potential difference between the outer ring and the inner ring of a bearing (hereinafter, referred to as a shaft voltage). The shaft voltage includes a high-frequency component caused by switching. When the shaft voltage reaches the dielectric breakdown voltage of the oil film inside the bearing, micro-current flows inside the bearing and causes electrolytic corrosion inside the bearing. When the electrolytic corrosion proceeds, a wavy abrasion phenomenon can occur on the bearing inner ring, the bearing outer ring, or bearing balls, which leads to abnormal sound. This is one of the major factors of failures in the electric motor.
In this manner, electric motors using the PWM system are likely to generate unnecessary high-frequency signals that cause noise, and such unnecessary electromagnetic radiation causes electrolytic corrosion in the bearing.
The conventional measures considered to suppress electrolytic corrosion are as follows:
(1) Providing electrical continuity between the bearing inner ring and the bearing outer ring;
(2) Providing electrical insulation between the bearing inner ring and the bearing outer ring; and
(3) Reducing the shaft voltage.
Examples of the specific methods for (1) include using a conductive lubricant in the bearing. However, the conductive lubricant has conductivity deteriorated with a lapse of time, and lacks sliding reliability. An alternative method considered is to dispose brushes on the rotating shaft so as to provide electrical continuity. However, this method produces brush abrasion powder and requires space.
Examples of the specific methods for (2) include replacing the iron balls in the bearing with non-conductive ceramic balls. This method is highly effective in suppressing electrolytic corrosion, but requires high cost. Thus, this method cannot be used for general-purpose electric motors.
As a specific method for (3), the following method is conventionally known. The stator iron core and conductive metal bracket are electrically short-circuited so as to change the capacitance and reduce the shaft voltage (see Patent Literature 2, for example).
In Patent Literature 2, short-circuiting the stator iron core and the bracket reduces the impedance of the stator side and thereby suppresses electrolytic corrosion in the bearing.
That is, generally, in the electric motor that is used in a washing machine, a dish washer/dryer, or the like, installed in a wet place and thus can cause electric shock, independent insulation (hereinafter, referred to as additional insulation) needs to be added to the insulation in the live part (basic insulation). In contrast, the electric motor used in an air conditioner has no danger of electric shock and thus requires no additional insulation.
Thus, in the electric motor used in an air conditioner, generally, the rotor does not have an insulated structure and the impedance of the rotor side (bearing inner-ring side) is in a low state. In contrast, the stator side (bearing outer-ring side) has an insulated structure and thus the impedance is in a high state. In this case, impedances cause a difference in voltage drop. The electric potential of the bearing inner-ring side is high while the electric potential of the bearing outer-ring side is low. This unbalanced state can produce a high shaft voltage. Such a high shaft voltage can cause electrolytic corrosion in the bearing.
In order to avoid such a state, in Patent Literature 2, the following method is used. The stator iron core and the bracket are short-circuited so as to eliminate the capacitance component between them. Then, as described above, the impedance of the stator side (bearing outer-ring side) is lowered and approximated to the impedance of the rotor side (bearing inner-ring side).
Particularly in the structure of a general brushless motor, the following phenomena and reasons for them are considered. For the impedance of the stator side between the stator iron core and the bearing outer ring, generally, the stator iron core and the bearing outer ring are not electrically connected and are disposed with a certain space, and thus the impedance between them is in a high state. Further, since the impedance between them is high, the high-frequency signal generated from the stator iron core attenuates and reaches the bearing outer ring. As a result, a high-frequency voltage at a low electric potential occurs in the bearing outer ring.
In contrast, for the impedance of the rotor side between the stator iron core and the bearing inner ring, the stator iron core faces the rotor with a small clearance provided between them, and generally the rotor and the rotating shaft are made of conductive metals, and thus the impedance between them is in a low state. Further, since the impedance between them is low, the high-frequency signal generated from the stator iron core reaches the bearing inner ring without attenuation. As a result, a high-frequency voltage at a high electric potential occurs in the bearing inner ring.
As described above, also the structure of the brushless motor itself is likely to unbalance the impedance of the rotor side and that of the stator side. This generates an electric potential difference, i.e. a shaft voltage, between the inner ring and the outer ring of the bearing, which causes electrolytic corrosion in the bearing. The major source of generating signals that cause such a shaft voltage is the stator iron core that has a winding to be driven by high-frequency switching based on the PWM system wound thereon. That is, since the winding to be driven by high-frequency current is wound on the stator iron core, in the stator iron core, high-frequency signals are generated by driving high-frequency waves as well as magnetic flux generated by the driving current. The generated high-frequency signals are led to the bearing inner ring and the bearing outer ring via space.
In recent years, a molded motor has been proposed. In this type of motor, fixed members, such as a stator iron core on the stator side, are molded with a molding material, for example, to increase reliability. Then, it is considered that the bearing is fixed by such an insulating molding material, instead of a metal bracket, so as to suppress the unnecessary high-frequency voltage generated on the bearing outer-ring side and the unnecessary high-frequency current flowing between the inner ring and the outer ring of the bearing. However, since such a molding material is made of resin, its strength is not sufficiently high to fix the bearing. Further, the resin molding provides low dimensional accuracy, which is likely to cause creep failures in the bearing. That is, generally in a bearing, when a gap is present between the outer ring and the inner circumferential face of the housing, for example, force in the radial direction is caused to the rotating shaft by the transfer load. When such force is generated, the relative difference in the radial direction is likely to cause a sliding phenomenon. Such a sliding phenomenon is called creep. Generally, such creep can be suppressed by securely fixing the outer ring to the housing, such as a bracket. Further, with recent increases in the output of an electric motor, more secure fixation of the bearing becomes necessary. Thus, it is essential to take creep-preventing measures, such as using a metal bracket preformed from a steel sheet with a high dimensional accuracy to fix the bearing. Especially, it is typical that bearings journal a rotating shaft at two points. It is preferable that two bearings are fixed by metal brackets for the reasons of the above-mentioned strength and easy implementation.
As a technique for suppressing electrolytic corrosion, the conventional method as disclosed in Patent Literature 2 has the following problem. That is, this conventional method is to suppress electrolytic corrosion by changing the impedance of the stator side and keeping the electric potential balance between the inner ring and the outer ring of the bearing. In such a method, if the impedance is unbalanced in a usage environment of the electric motor, the following case can be considered. That is, inversely, the shaft voltage is increased and electrolytic corrosion is more likely to occur.
Particularly in the case of an air conditioner as an indoor unit, a heat exchanger generally made of metal makes up a sizable proportion of the inside of the air conditioner. Further, a fan connected with a rotating shaft is disposed so as to face this heat exchanger. Thus, large capacitance is likely to be generated between the heat exchanger and the fan. Thus, in the case of such an air conditioner, the effect of this capacitance can unbalance the impedance in the electric motor with high possibility. That is, for example, high-frequency signals radiated from the stator iron core flow into the rotating shaft as high-frequency current via the heat exchanger and the fan. This can further increase the electric potential on the bearing inner-ring side.
When two bearings are fixed by metal brackets for the above reason of strength, the impedances of both brackets are different because one bracket and the other bracket have different shapes and are disposed in different positions. Thus, the electric potential induced in the one bracket is different from the electric potential induced in the other bracket. This causes different shaft voltages in the two bearings and can cause a failure such that one bearing has no electrolytic corrosion but the other bearing has electrolytic corrosion.
PTL1
    Japanese Patent Unexamined Publication No. 2007-198628PTL2    Japanese Patent Unexamined Publication No. 2007-159302